The Basics
Name: What's their name, why did their parents choose it, does it have any significance?
Appearance: Nothing too complex like golden locks complimenting bright blue eyes with a black ring around them around a darker than normal skin (more like: blonde hair, blue eyes, hispanic)
Age: How old are they, how old are they compared to the rest of the characters?
Affiliation: Previous affiliation before coming to (soon be named) army?
Appearance: Hair (Style, Length, Colour), Eyes, Body type, Skin Tone, Noticeable scars ect
Clothing Choice: Readers HATE reading long expositions about clothing choice so generalities and practicality are things to keep in mind.
Life Prior to Military: What was it like growing up, did it affect them in some way? What was their family life like? Education? Black marks against their name?
Personality: Detailed! To start you off try typing [Name] is [Character Flaw] Aaron is a grouch... Ohio is a yappy little terrier, always picking fights... Lisa is a total showoff... Then build around that main character trait! The more detailed, the more 3d and realistic they feel to a reader.
Fatal Flaw: How can they screw up a mission and make people hate them?
(Ex. Annoying, Braindead, Skirtchaser, over badass, easy, over virtuous)
A character with flaws reminds us of ourselves, that we empathise with the characters more.
Saving Virtue: How can they occasionally save the day? What personality trait compliments?
Fears and Dislikes: Will they come into play during the story and bugger things up? Don't mention things if they don't matter to the story.
Quirks: What is something odd about them and may cameo or contribute to the story or plot? (Aka colorblind or something)
Talents: What are they good at that isn't related to things on the battlefield that might come into play during the story? Can they tie knots? Good at climbing?